The present project is to investigate the cause(s) of B12 malabsorption in enteric overgrowth syndromes with particular emphasis on the production of B12 analogs by micro-organisms. Four factors (A,B,I, and pseudo-B12) have been identified within the intestinal lumen and in host liver demonstrating production and absorption. Present studies are designed to demonstrate whether conversion of injected B12 occurs, or synthsis de novo is the principal mode of analog production. The ability of the analogs to compete for B12 binding sites on intrinsic factor and mucosal binding sites is also under study. To date, it appears that analog accumulation occurs concomitantly with depletion of hepatic stores of B12.